What you are doing won't require 1000 dollars your activities are not very demanding anyway,you want to go with a quad since that seems to be the way of the future right now, you can build a nice phenom 2 x4 955 set up for that price,you won't need cross fire (honestly even the 955 is overkill) you want a nice graphics card that will last you a while your resolution isnt very high....honestly i'd save some money by buying a tri core pheom 2 x3 720 BE,a 4870 or gtx260,a western digital raptor for your operating system/programs and a large drive perhaps a 640 gb or 1 TB drive for your music and movies, you should not worry about crossfire or sli but thats just me i prefer single card solutions that way i don't have to spend too much on power supplies,get a nice aluminum case from cooler master or lian li, and then the money you save use it to get a new and better monitor (a gamer in 2009 should not be playing on nothing less than a 1680x1050 resolution) all that should fit within your budget if you plan on buying your parts from newegg.com....you can also go the intel route but their 775 socket wont receive any upgrades and the i5 does not come out till later so amd is the best price to performance at this point. just my two cents.
Comes out to $1038 but, if you add in Mail-in rebates, it should be below $1k.
If you wanted it lower, that would include in taking crossfire option away and getting a 650tx instead or the other option is, getting the 4870 1gb sapphire, keeping the 750tx and being able to crossfire later.
now, I know you only have a 1280x1024 so, in the near future, look to upgrading that aswell. I'd probably drop the 4890 and go with the 4870 instead.
I was also thinking of switching out the x4 955 since it's only gaming is what you asked for and getting the x3 720 pII combo with a 790gx motherboard. Probably also another best bang for your money aswell.